John,
It really depends on how picky you are. Particularly with slides (and especially if they're Kodachromes) it takes a very good scanner to dig out the details. Any consumer flatbed (even one that says it does film and tries to lure you with outrageous resolution and DMax numbers) is simply going to fall short, due to lack of
true, as opposed to spec'ed resolution and dynamic range.
You really want a dedicated film scanner if you're particular. If you're not as picky, one of the top-line flatbeds may be good enough for you. Either way, expect to spend some time and do some reading while learning to scan. Getting top quality out is, as I mentioned before, a tedious and fiddly process.
If you've got a fair number of 35mm slides, the best bang for the buck will come with the Nikon Coolscan V. They're not cheap, but not outrageously expensive either. The other alternative is to go through your collection, pick the ones you want the most and pay someone else to get 'em digitized.
If you've got any detailed or specific questions, I'd be happy to answer them backchannel. I've more experience doing this than I care to think about. :-)
Scott